The intracellular administration of drugs is feasible if we encase them in liposomes (lipid vesicles), or combine them with large molecules, which are taken into cells by phagocytosis. They become localized in lysosomes whose digestive activity liberates the drug. Because this new method of drug delivery is promising for cancer chemotherapy, we propose to seek a practical model system in which the process of uptake and drug release can be conveniently studied. Fluorogenic substrates for lysosomal enzymes will be used to visualize uptake. Different types of liposomes will be studied as to their penetration into various cells and tissues, and methods of enhancing and targeting their uptake will be investigated.